1. Field of the Invention
The present invention concerns a device for doping silica powder. Generally speaking, the powders obtained with a device of the invention will be usable in any type of application, and more particularly for the fabrication of optical fiber preforms.
2. Description of the Prior Art
Doping is an operation which consists of incorporating into the silica molecules one or more molecules of elements intended to modify their properties. For example, in the field of optical fibers, dopants are incorporated into the silica to modify its refractive index, respectively increasing it or reducing it according to whether the silica concerned is to form the core or the optical cladding of an optical fiber, for example. The dopant can be germanium to increase the refractive index of the silica or fluorine to reduce it, for example.
Silica ready for use in the fabrication of optical fibers is in the form of a powder made up of grains of greater or lesser size, and possibly densified.
For example, densified silica grains are used for plasma coating of an optical fiber preform fabricated by the modified chemical vapor deposition (MCVD) process.
Silica grains that have not been densified are usually smaller in size than densified silica grains and can be used to fabricate a preform by the vapor axial deposition (VAD) or outside vapor deposition (OVD) process, for example.
All the above processes are mentioned merely by way of example. The invention is in no way limited to these methods, which are well known to the skilled person in the field of optical fibers and will not be described in more detail here.
The fabrication of densified silica grains begins with non-densified silica grains in the form of very small particles, generally with a size between 0.1 nm and 100 nm. In this form the silica powder is called soot. The silica soot can be obtained in various ways well known to the skilled person. For example, it can be made by oxidizing in the presence of heat a silica precursor gas, such as silicon tetrachloride SiCl.sub.4. To fabricate densified silica grains the small silica particles are agglomerated to form granules and these granules are then consolidated by heating them to eliminate the pores between the particles constituting them, with the result that the grains obtained in this way are dense. These grains generally have a size in excess of one micron.
The expression "silica granule" refers to a silica particle at an intermediate stage of the process of fabricating densified silica grains.
The conventional method used to obtain a powder made up of doped silica grains consists in carrying out the operation of densifying the granules in an atmosphere containing a precursor gas of the required dopant. For example, to fluorinate silica granules densification is carried out in an atmosphere containing a fluorine-containing gas such as sulfur hexafluoride SF.sub.6 or silicon tetrafluoride SiF.sub.4. The non-densified silica granules are placed in a crucible that is placed in a furnace to heat it to the temperature at which densification can be effected, the furnace being fed with a precursor gas of the required dopant. The doping is produced by F.sub.2 dopant molecules diffusing into the silica granules, which leads to the formation of complex molecules of the SiO.sub.2-x F.sub.2x type.
Crucible type devices for implementing this type of process are subject to a number of problems.
The main problem is that, to obtain homogeneous doping, the treatment time required is very long and the doping yield is mediocre, which represents a penalty. If the treatment time is reduced, which increases the yield, the doping is not homogeneous. Non-homogeneous doping of silica grains leads, in the application to the fabrication of optical fibers, for example, to index variations that result in unacceptable transmission performance of the optical fibers.
Another serious problem is that the doping is effected in a static manner and therefore requires a first phase in which the temperature is raised in the furnace, a second phase of treatment at substantially constant temperature and a final phase in which the temperature is reduced before the doped and densified grains can be recovered. This means that the process cannot be continuous, which also represents a penalty in terms of the yield.
One aim of the present invention is to develop a device for doping silica powder that is capable of homogeneous doping, compatible with the use of the silica in the fabrication of optical fibers and without any penalty in terms of the yield of the doping operation.
Another aim of the present invention is to develop a device for doping silica powder enabling continuous doping.